What to do when an ex boyfriend says that he isn't happy without you and that he made a bad choice breaking it off, but won't get back together?

My ex boyfriend recently broke up with me. He said he just didn't understand me and he broke things off. He still wanted to be friends, so we are. But every now and then, he brings up how unhappy he is without me and that he made the wrong decision, but he doesn't want to get back together. He keeps bringing this up and most of the time I don't know what to say.